Timber, as a natural biological material, shows considerable variation in its properties. This variation occurs both from one tree to another and within a single board of timber. Given the widespread use of timber as a structural component, it is important economically to be able to test it quickly and reliably for sorting into groups with guaranteed engineering properties and/or appearance. The use of microwave radiation for the testing of timber meets these requirements of economy, speed and reliability--being non-destructive and allowing non-physically contacting test apparatus to be built which is particularly suitable for automated real time monitoring of timber in high speed saw milling operations.
Furthermore, microwave radiation can be used, reliably, to determine various parameters affecting the physical and structural properties of wood. For example, the loss, phase shift and depolarisation of microwaves transmitted through wood (which is a strongly anisotropic substance) can be measured to infer moisture content, density, presence of knots, presence of pith, slope of grain and parallelism of fibres. A theoretical background for such measurements is given in the papers "Microwave Electromagnetic Non-destructive Testing of Wood in Real Time" Y. Yen 1981, University Microfilms International, Michigan, USA; and "Microwave Electromagnetic Non-Destructive Testing of Wood" by R. J. King, Proceedings of Symposium on Non-destructive Testing of Wood, Vancouver 1978, pages 121-134.
Known microwave apparatus for testing wood, as disclosed in the above two papers, involves an arrangement wherein a microwave signal is transmitted through a timber sample to be scattered by a relatively complex mechanically spun, electrically modulated dipole. A portion of the resultant scattered signal propagates back through the timber and is received by the same antenna from which the original signal was transmitted for mixing with a reference signal and subsequent processing. Measurements of loss, depolarisation and phase shift are made and the characteristics of the timber sample such as density, moisture content, grain direction, parallelism of fibres and presence of pith are derived therefrom.
U.S. Pat. No. 4,123,702 and GB patent No. 1489554 disclose systems wherein knots in timber are detected by transmitting frequency modulated microwave radiation through the timber and detecting phase shifts in the transmitted microwaves--the phase shifts being caused by the presence of knots. GB patent No. 1564194 discloses knot detection apparatus requiring at least one pair of transmitting antennae and in which 180.degree. phase shifted microwave signals are transmitted through or reflected from mutually adjacent areas of the timber and are then compared, a null output indicating the absence of knots. GB 1564194 also discloses detection of deviations of the timber grain direction by measuring phase and/or amplitude changes due to depolarisation of the 180.degree. out of phase incident signals (the direction of polarisation of the incident signals being different). GB patent No. 1560591 discloses a similar system in which there is a single transmitting antenna and at least one pair of receiving antennae. The apparatus disclosed in U.S. Pat. No. 3,810,005 also depends on a comparison between microwave radiation measured at two adjacent positions, after its transmission through the timber. This system requires the presence of two detectors.